1. Field of the Invention
This invention relates to drive circuits for providing turn-on and turn-off control of switching devices such as power transistors normally carrying relatively high values of output current in the on condition. Such drive circuitry is commonly employed as a portion of DC-AC or DC-DC converters.
2. Description of the Prior Art
Transformer-coupled base drive circuits are well known in the art and generally feature regenerative current drive for power transistor turn-on wherein the output current of the power transistor is transformer-coupled back to the base circuit of the power transistor.
In a pending application by Burens et al, also assigned to North Electric Company, Ser. No. 476,254, filed June 4, 1974, now U.S. Pat. No. 3,930,170, a base-drive circuit is disclosed wherein a reverse sweep-out current is provided to the power switching device at turn-off by means of a resistor-capacitor biasing arrangement. However, there is a core reset problem with this and many other prior art arrangements under certain transient conditions which force the transformer core into eventual positive saturation in such a manner as to cause catastrophic failure in the power switching device being controlled by the transformer-coupled base-drive arrangement.
One prior art approach to the core reset problem is found in the Higgins U.S. Pat. No. 3,610,963. This arrangement provides a core reset action prohibiting positive saturation of the transformer core. However, the Higgins approach provides a short circuit rather than a reverse voltage to the base of the power switching device at the initiation of turn-off resulting in much longer turn-off times and inefficient power losses. Provision of a short circuit, as in the Higgins patent, is ineffective in reducing leakage current in the turned-off power device. Also, in the absence of a reverse bias voltage, the power device is more susceptible to false triggering by noise.